Methods and devices for detecting at least a portion of a gas in a gas mixture are known from the related art. The present invention is described below, without limitation of further possible embodiments, essentially with reference to devices that are used for quantitatively and/or qualitatively detecting at least a portion in particular of a partial pressure and/or a volume portion and/or mass portion, of a gas in a gas mixture. For example, the gas may be an exhaust gas of an internal combustion engine, in particular in the automotive field. The sensor for detecting the gas portion is in particular a lambda sensor. Lambda sensors are described, for example, in Konrad Reif, Ed., Sensoren im Kraftfahrzeug [Automotive Sensors], 2nd Edition, 2012, pp. 160-165. Lambda sensors, in particular universal lambda sensors, place two mass flows, in particular oxygen mass flows, between two spaces, which may be a gas chamber outside the device or a cavity in the device, into an equilibrium. One of the mass flows is driven across a diffusion barrier due to concentration differences. Another mass flow is driven across a solid-state electrolyte and two electrodes, in particular two pump electrodes, controlled by an applied pump current. The pump current is preferably adjusted in such a way that a constant, very low oxygen concentration results in the cavity. A concentration profile across the diffusion barrier is uniquely determined by a constant control point in the cavity, in particular a constant setpoint voltage that results in an oxygen concentration, and by an oxygen concentration on the exhaust gas side. An inflow of oxygen molecules from the measuring gas chamber to the cavity results, corresponding to this unique concentration profile, and corresponds to the adjusted pump current. Therefore, the pump current may be used as a measured value of the oxygen concentration in the measuring gas chamber, in particular the oxygen concentration present on the exhaust gas side.
Furthermore, methods and devices are known for detecting at least a portion of the measuring gas component containing bound oxygen in a gas mixture, in particular in an exhaust gas of an internal combustion engine, by detecting a portion of oxygen that is generated by a reduction of the measuring gas component containing the bound oxygen, in the presence of molecular oxygen. European Published Patent Application No. 0769693 provides a method and a device for detecting at least a portion of the measuring gas component containing bound oxygen, in particular nitrogen oxides NOx, in a gas mixture by detecting a portion of oxygen that is generated by a reduction of the measuring gas component containing the bound oxygen, in the presence of molecular oxygen, in particular by the reduction of the nitrogen oxides NOx with the aid of a catalyst that is suitable for this purpose. The device described therein includes a first pump cell which adjoins a first cavity that is connected to the measuring gas chamber, the first pump cell being used for transporting oxygen from the first cavity, resulting in a lower oxygen partial pressure in the first cavity. The device also includes a reference cell which adjoins a reference gas chamber and is used for transporting oxygen from the second cavity in such a way that an oxygen partial pressure in an atmosphere in the second cavity may be regulated in such a way that the oxygen partial pressure has a value which essentially does not adversely affect the detection of the portion of the measuring gas component. Lastly, the device includes a second pump cell which adjoins a second cavity, whereby an electrode which adjoins the second cavity, in particular due to a catalyst introduced into the electrode for this purpose, is configured for reducing or decomposing the measuring gas component containing the bound oxygen, preferably nitrogen oxides NOx, in the atmosphere introduced into the second cavity. The oxygen which is generated by reduction or decomposition of the measuring gas component in the second cavity, and which preferably originates from the reduction of the nitrogen oxides NOx, is transported into the reference gas chamber with the aid of the second pump current, and its proportion is detected using a value to which the second pump current is set. The portion of nitrogen oxides NOx in a gas mixture, which in addition to the nitrogen oxides NOx also includes oxygen, in a measuring gas chamber may be determined in the described manner with the aid of a cascade of at least three pump cells situated in succession.
Devices for on-board diagnostics (OBD) of motor vehicles are also known from the related art. By use of OBD, in particular components of an internal combustion engine in a motor vehicle that have an influence on a characteristic of an exhaust gas of the internal combustion engine may be monitored. With regard to a characteristic of the exhaust gas, faults that occur are detected by the OBD and indicated to the driver of the motor vehicle via an indicator light, for example, and permanently stored in an associated control unit. OBD was first introduced in 1988 by the California Air Resources Board (CARB) against the background that compliance with the exhaust gas emission limits should be ensured not only at licensing of the motor vehicle, but also over its service life. In the further development of the second generation of the vehicle onboard electronics systems for self-monitoring (CARB OBD II) presently in use, there are requirements, inter alia,—in contrast to known electronic control devices—for accurately determining and locating malfunctions of the exhaust gas-relevant systems, for which the term “pinpointing” is also used.
In this regard, variations in output currents from current sources, which may have tolerances of up to 30% or greater, play a special role. As the result of manufacturing-related tolerances of the components used for the current sources, together with additional effects, in particular due to aging of the components and/or due to temperatures to which the components are exposed, the output currents of the current sources may differ significantly from their nominal values. However, as discussed above, since the oxygen partial pressure in the reference cell is proportional to the magnitude of the reference pump current, the resulting variations in the reference pump current may have quite major effects on the characteristics and functioning of the devices for detecting at least a portion of a measuring gas component in a measuring gas chamber. If the reference pump current differs from an established setpoint value in this way, the measuring accuracy of the known sensors for detecting the gas portion may be greatly limited.